(a) Field of the Invention
The present invention concerns a semiconductor device having a p type semiconductor region made with a II-VI compound semiconductor material, and more particularly it relates to such a semiconductor device whose p type semiconductor region is obtained by doping a Group Ia element of the Periodic Table into a II-VI compound semiconductor material.
(b) Description of the Prior Art
Such Group II-VI compound semiconductor materials as ZnS, ZnSe, CdS and CdSe, as is well known, are direct band gap semiconductors as in the case of GaAs which is one of Group III-V compound semiconductors in which the lowest conduction band minimum and the highest valence band maximum are both at a same wave vector in the Brillouin zone. That is, Group II-VI compound semiconductor materials are of the direct transition type with respect to electronic transition as in the case of GaAs.
In addition to the property mentioned above, II-VI compound semiconductor materials, in general, are characterized by their wide energy band gaps. Thus, they are considered to be attractive as semiconductor materials in that they allow us to expect properties which cannot be obtained by III-V compound semiconductors including GaAs.
However, owing to various reasons including the difficulty of obtaining stoichiometry of II-VI compounds, studies concerning II-VI compound semiconductors have not made much development yet as compared with the studies of III-V compound semiconductors. Therefore, it is the present status of the art that the characteristics of II-VI compound semiconductors have not been sufficiently utilized in the field of semiconductor devices.
Detailed reasons therefor will be described later, but the general reason is that it is relatively easy to produce II-VI compound semiconductors with the n type conductivity, however it is rather difficult to produce p type crystals. It should be understood, of course, that conversely there are those II-VI compound semiconductors from which p type crystals can be made relatively easily but for which the growth of n type crystals is not easy. Those semiconductor devices manufactured at present with II-VI compound semiconductors are represented typically by photoconductors which use a semiconductor bulk having either one of the conductivity types. Those devices having an impurity-controlled pn junction formed with a same substrate have not yet reached the stage of being actually practiced. If, as in the case of the semiconductor devices using III-V compound semiconductors, the above-mentioned difficulty which prevails at present is overcome, and if there can be obtained, at will, II-VI compound semiconductor materials having either the p type or the n type just by controlling the doping of the impurity elements, it is certain that the scope of their use and application will become drastically broadened.
Some discussion will be made hereunder of the present state, as well as, the problems of the II-VI compound semiconductors.
The energy band gaps of reliable respective crystals of II-VI compound semiconductors which have been reported and their conductivity types which are obtained according to the ordinary known techniques are shown in the following Table 1.
TABLE 1 ______________________________________ ZnS ZnSe ZnTe CdS CdSe CdTe ______________________________________ Conductivity n n p n n n .multidot. p type Energy band 3.6 2.67 2.2 2.5 1.74 1.5 gap E.sub.g (eV) ______________________________________
As will be understood from Table 1, the crystals of II-VI compound semiconductors are such that there can be obtained crystals having only either one of the n type and p type, except for the CdTe crystal. Especially, ZnSe crystal has a large energy band gap: E.sub.g of 2.67 eV, as compared with that of GaAs (its E.sub.g is about 1.43 eV) and with that of GaP (its E.sub.g is about 2.26 eV) of the III-V compound semiconductors. Thus, if a diode having a pn junction can be manufactured with a ZnSe crystal, it will be a semiconductor material suitable for obtaining a blue light emitting diode whose light emission spectra at room temperature is in the blue range. This could not have been realized in the past. However, as also shown in Table 1, the level of technology has not yet risen high enough as to provide crystals of both the p type and the n type at will just by controlling the conductivity types of the crystals. Therefore, efforts are being made energetically at various laboratories throughout the world to realize the above-mentioned purpose, and various reports have been presented so far with respect to the formation of pn junctions.
Among the recent reports conveying the success in the formation of a pn junction, a relatively reliable one concerns the acquisition of a pn junction by using an n type ZnSe crystal obtained by the usual manufacturing method, and by the ion implantation into this n type ZnSe crystal, of either phosphorus (P) atoms or arsenic (As) atoms which serve as a p type impurity by its substantial occupation of the lattice sites of the Se atoms, by relying on the ion implantation technique.
However, a study has been made of the characteristic of light emission spectra of the light-emitting diode (LED) thus obtained, by causing a forward current to flow through this diode. It has been found that the light emission due to recombination via the deep level is dominant. There has not been observed light emission due to recombination corresponding to the value of the energy band gap, or of a value close to the energy band gap.
The problems of II-VI compound semiconductors stated above are considered to be due roughly to the following two reasons.
The first concerns imperfection of the semiconductor crystal. In such II-VI compound semiconductors as ZnS, ZnSe, CdS and CdSe, Group VI elements in general have a higher vapor pressure than Group II elements. Accordingly, in crystals which are obtained by a growth step conducted at a high temperature, it will occur that, during the growth of the crystal, the Group II element having a high vapor pressure will become scarce due to its vaporization out of the crystal, thereby resulting in a deviation from stoichiometric composition. More particularly, it is usual that vacancies of the Group VI element develop in a large number. These vacancies act as the donor. Accordingly, only an n type crystal is obtained ordinarily, and no practical p type crystal can be obtained easily. Thus, as a matter of course, it is difficult to produce a device having a pn junction. Vacancies of the Group VI element tend to any combination of the impurity to form complex centers, and these complex centers will become non-radiative recombination centers or deep levels. Accordingly, even when a pn junction is formed, an LED having such pn junction will have a very weak light emission efficiency, or the light emission from deep levels will become dominant.
As such, in order to obtain an LED capable of providing a light emission corresponding to the value of the energy band gap by the use of a II-VI compound semiconductor material, there has to be formed a pn junction with a crystal having a high crystal perfection not containing either such non-radiative recombination centers or deep levels. Demand has been presented for the development of a technique to form such a desirable pn junction as mentioned just above.
The second important reason is that the following demand has not been satisfied yet. That is, when an impurity element for determining the conductivity type of the crystal is doped in a II-VI compound semiconductor, the level of such impurity as will become either the donor level or the acceptor level which is formed in the forbidden band is positioned in the bottom of the conduction band or close to the top of the valence band, i.e. it has been desired to be a shallow impurity level.
As the impurity elements for producing the p type conductivity in II-VI compound semiconductors, there are considered, for example, Group Ia or Ib elements which substantially occupy the lattice sites of the Group II element, or Group Vb elements which substantially occupy the lattice sites of the Group VI element. Thus, these elements are expected to render the conductivity type of the crystal to the p type. There have been reported many results of experiments of impurity doping which rely on either the ion implantation technique or the diffusion technique. The result obtained on ZnSe crystals as well as the result of use of n type impurity elements are shown in Table 2.
TABLE 2 ______________________________________ Impurity levels of various impurity elements in ZnSe crystal Impurity Level Impurity Conductivity Acceptor Level or element type Donor Level (eV) ______________________________________ Li p 0.66 Li p 0.114 Na p 0.085-0.100 Na p 0.09 Cu p 0.072 Cu p -- N p 0.136 P p 0.68 P p -- Ise p 0.68 Vzn p .about.0.1 Ga .multidot. In p -- Al n 0.026 Al n -- Ga n 0.028 Ga n -- In n 0.029 F n 0.029 Cl n 0.027 Cl n -- ______________________________________
As will be apparent from the result of the p type impurity element mentioned, it is noted that, in any one of these impurity elements, there is formed an impurity level having a deep level, and the technique is considered to represent a doping technique which is not suitable for the fabrication of a desired semiconductor device. The reason therefor is considered to be that because no good crystal has been grown, the doped impurity cannot form a shallow level by itself alone no matter how much impurity is doped in the crystal, so that the impurity as well as such crystal defects as vacancies will tend to combine together to develop complex centers which will become deep levels.